This invention relates generally to apparatus for evaluation of geothermal wells and more specifically to apparatus for collecting subsurface liquid or steam samples from geothermal drillholes.
In one type of geothermal energy development project, a borehole is drilled into a region of low-permeability, hot crystalline rock and a large hydraulically fractured area is produced from the bottom of the borehole. A second borehole is directionally drilled to intersect the fractured region. Pressurized water is pumped down one borehole, circulated through the fractured area of the formation to recover heat from the rock, and returned by way of the second borehole to the surface where energy is extracted.
In the development of geothermal energy it is highly desirable to evaluate borehole and formation characteristics. One such desirable measurement is to sample the liquid or steam present within the geothermal borehole. Chemical analysis of discrete borehole samples have proven useful in the determination of reservoir properties. Such samples enable accurate assessment to be made of the water or steam composition or steam quality providing associated correlations with rock mineralogy, temperature and pressure.
Apparatus for retrieving samples of borehole fluid have been well known in the oil and gas industry for many years. Devices of this class have been tried and have been proven unsuitable for sampling in geothermal boreholes. Standard oil and gas borehole samplers have been unable to endure in geothermal environments, due to limitations with respect to time-temperature exposure capacity. A sampler for use in a geothermal environment must be capable of sustained operation at temperatures above 200.degree. centigrade. This temperature range requirement is considerably above the upper operating temperature limit of standard oil and gas borehole fluid samplers. Samplers designed specifically for use in the geothermal environment have fallen into two broad classes: (1) bottles initially open which close after sampling and (2) bottles which remain closed throughout descent and ascent and open only for sample collection. A typical sampler of class one would utilize ball valve seals at the upper and lower sample chamber openings. As the apparatus descends the borehole, both valves are held unsealed by flow drag created by descent. Upon retrieval the upper ball valve seals under the weight of the ball plus pressure created by the retrieval velocity of the sampler. In ascent, the lower valve seal is sealed under the weight of the ball alone. Samplers of this general type, while mechanically simple and able to operate in high temperatures, have proven unsatisfactory. With this design, there in uncertainty as to the time of ball valve sealing and uncertainty as to whether the seal remained intact during retrieval from the borehole. A further shortcoming is the difficulty of adequate flushing of the sample chamber prior to sampling, due to the relatively small valve apertures through which purging of the sample chamber must occur during descent.
A particular prior art sampler of the second class utilizes an electronic of mechanical clock, a trigger unit and linkage connecting the trigger unit to a valve. In operation, when the clock fires the trigger unit opens the valve sealing the sample chamber allowing a sample of borehole fluid to be taken. When the pressure within the chamber equalizes with the borehole pressure the valve shuts, sealing the sample within the chamber. While representing a considerable improvement over samplers of the class one type, samplers using clockwork mechanisms have not proven reliable in high temperature operations and have an excessive servicing requirement.
Another sampler of the class two design utilizes an inertia mechanism combined with a break-off tube to eliminate clock mechanisms. Upon arrival at the sampling location within the borehole the wireline suspending the sampler in the borehole is vigorously shaken. The oscillation of the inertia mechanism causes a striker to shatter a break-off tube allowing a fluid sample to enter the sample chamber through non-return valve. As the sample chamber fills and the pressure within the sample chamber equalizes with the borehole pressure the valve closes. As is apparent from the above description, an operator at a surface location can never be confident that he has oscillated the instrument enough to assure the break-off tube has been shattered. Thus, while this sampler meets environmental requirments, it is less than completely reliable in retrieving a sample of the liquid or steam within the borehole.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a simple mechanical apparatus for the reliable collection of liquid or steam samples from the hostile environment of a geothermal borehole.